In a papermaking process a number of papermaking chemicals are used for process control and to give required properties to the paper. The papermaking chemicals are dosed to the wet-end of a papermaking machine, by incorporation into an aqueous fibre suspension before it is fed from the headbox to the forming fabric. The goal is to have the chemicals adsorbed onto the surface of fibres by electrostatic forces.
The main difficulty in the simultaneous use of several papermaking chemicals, which are adsorbed on the fibre surface by similar mechanism, is how to achieve quantitative retention and an even distribution on fibre surface. Almost all additives have to compete for the free bonding (anionic, cationic, and neutral) sites on fibre surface. In most cases this leads to incomplete retention and/or uneven distribution of the chemicals on the fibre surfaces. As a result the quality of the finished paper suffers, and runnability problems will occur in the paper machine. In addition to inadequate retention and distribution, simultaneous use of several reactive additives may cause harmful interactive reactions between various papermaking chemicals and thus decrease their functionality and effect.
To improve the retention of papermaking chemicals as well as fines present in the fibrous suspension (amount of materials retained in the web being formed) a number of specific papermaking chemicals (retention chemicals) are conventionally used. The papermaking chemicals with a low retention to the fibre surface are accumulated in the white water system and can stick to paper machine surfaces as dirt, or to each other forming agglomerates. Such agglomerates can cause web breaks and dirty spots to the paper that is produced. Contrary to that good retention reduces the amount of fibre, filler and other chemicals passing to the paper machine short circulation and accumulating in the process system.
Papermaking chemicals which are used in high amounts are the main reason for harmful dirt precipitations in the paper machine and the resulting runnability and quality problems. Such papermaking chemicals include for example sizes, fillers and wet and dry strength giving chemicals.
The mechanism of chemical retention is that small particles (for example filler particles) are bonded as larger flocks, which the wet fibre web on the forming fabric can sustain. This flocculation can be achieved by use of different retention chemicals, which in most cases are water soluble polymers, polyelectrolytes.
In dual polymer systems two polyelectrolytes are used at the same time. Their difficulty in practice is that optimal conditions are hard to find and small process changes can affect a lot. Such dual systems work by having a short chain length polymer adsorb filler particles to its surface and thus form bonding points for a long chain polymer. In the first stage flocculation happens via mosaic formation and in the second stage by bridging.
Typical microparticle systems are for example:                cationic starch/polyacrylamide +colloidal silica (for example the one which is sold under trademark “Compozil”)        polyacrylamide +bentonite (for example the one which is sold under trademark “Hydrocol”)        
As a first step of such a prior art process cationic polymer is added to the paper making pulp, and then just before the headbox very fine (particle size 250 nm-10 μm) and in most cases highly negatively charged (about 1 meq/g) microparticles are added. Microflocs are thus formed, and these have strong flocculation tendency even after the flocks have once been broken down. This can be seen in that the white water has a strong capacity to flocculate. Flocks which are formed are (compared to traditional retention chemicals) very small and this effect is even increased by the after flocculation. Flocculation in micro scale gives a high porosity to the web and thus dewatering is improved, the solids content after the press section is increased, and drying energy need is reduced.